1. Field of the Invention
The present invention relates to a liquid crystal display and a drive circuit thereof that is suitable for placing a data line drive circuit having D/A conversion circuits only on one side of a panel for dot inversion drive.
2. Description of Related Art
In a well-known liquid crystal display, a polarity of a voltage applied from a data line to a pixel via a TFT (hereinafter referred to as a pixel voltage) is inverted after each prescribed period. That means that pixels are AC driven. The polarity here indicates a positive or negative polarity of a pixel voltage with respect to a voltage of a common electrode of the liquid crystals (com voltage) as a reference. Such a drive method uses for inhibiting the degradation of liquid-crystal material.
For the drive method, there are known methods such as dot inversion drive method and 2H dot inversion method. In the dot inversion drive method, polarities of pixel voltages are inverted by adjacent data lines and scan lines so that adjacent pixels have different polarities each other. In 2H dot inversion drive method, polarities of pixel voltages are inverted by each adjacent data line and by two scan lines. These drive methods help reduce flicker, thereby improving picture quality.
Japanese Unexamined Patent Application Publication No. 8-129362 discloses a circuit in which one D/A conversion circuit drives a plurality of data lines in a time-sharing manner. In the drive circuit disclosed in this technique, odd-numbered data lines are connected to an upper data line drive circuit, while even-numbered data lines are connected to a lower data line drive circuit. In a given horizontal period (also referred to as a scanning period), a positive polarity analog video signal is outputted from the upper data line drive circuit at the same time when a negative polarity analog video signal is outputted from the lower data line drive circuit. Then, during a next horizontal period, a negative polarity analog video signal is outputted from the upper data line drive circuit at the same time when a positive polarity analog video signal is outputted from the lower data line drive circuit. This is how the dot inversion drive method is achieved. The drive circuit further includes an initialization circuit for initializing data lines to a com voltage during a horizontal blanking period, in order to drive in a time-sharing manner by controlling writing time and order. A gradation voltage provided from outside the data line drive circuit is inverted by each horizontal period. Therefore switch groups for selecting gradation voltage are constituted of high-voltage devices. Japanese Unexamined Patent Application Publication No. 2004-258485 discloses a configuration for RGB time-sharing drive.
However we have now discovered that there are some problems in the conventional circuit described as above. A first problem is that an area is required for placing data line drive circuits on an upper and a lower side of a panel. This causes a size of the panel to be larger. Consequently the number of panel to be retrieved from one sheet of mother glass is reduced. Moreover a larger area is needed for a flexible substrate wiring that supplies signals and power to the data line drive circuits.
A second problem is that a circuit area is expanded because switch groups for selecting gradation voltage are constituted of high-voltage devices. Having a high power supply voltage usually requires withstand pressure of devices constituting a circuit to be high. For this reason, a thicker gate oxide film Tox and a longer gate length L are needed, requiring a larger circuit area.